题目十六：合并两个排序的链表

题目描述：
输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。

解题思路：
递归版本

    public ListNode Merge(ListNode list1,ListNode list2) {

        if (list1 == null) {
            return list2;
        }

        if (list2 == null) {
            return list1;
        }

        if(list1.val <= list2.val){
            list1.next = Merge(list1.next, list2);
            return list1;
        }else{
            list2.next = Merge(list1, list2.next);
            return list2;
        }
    }

非递归版本

     public ListNode Merge(ListNode list1,ListNode list2) {

        if (list1 == null) {
            return list2;
        }

        if (list2 == null) {
            return list1;
        }

        ListNode mergeHead = null;
        ListNode current = null;

        while (list1 != null && list2 != null) {

            if (list1.val <= list2.val) {

                if (mergeHead == null) {
                    mergeHead = current = list1;
                } else {
                    current.next = list1;
                    current = current.next;
                }
                list1 = list1.next;

            } else {

                if (mergeHead == null) {
                    mergeHead = current = list2;
                } else {
                    current.next = list2;
                    current = current.next;
                }
                list2 = list2.next;

            }

        }

        if (list1 == null) {
            current.next = list2;
        } else if (list2 == null) {
            current.next = list1;
        }

        return mergeHead;
    }